This invention is directed to an apparatus and a high speed process for making a highly stretched film. A stretchable film is fed through the apparatus at a rate of up to 2,500 fpm wherein the film is heated and stretched by about 100-450% of its initial length, to an ultimate length that is about 200-550% of the initial length.
Prior art processes for stretching a film include processes wherein the film is heated before it is mechanically stretched, known in the art as xe2x80x9chotxe2x80x9d processes. Hot processes may be either in-line or off-line. Prior art stretching apparatus used in hot processes comprise a substantial number of stretch rolls. Further, these stretch rolls are spaced at large distances from successive rolls. As the film is fed through the apparatus, the film has minimum contact with the stretching rolls. As a result, the stretched film exhibits a considerable amount of neck-in as well as wrinkles, tears and/or holes. Hot processes are limited by moderate draw or stretch ratios, slow process speeds and temperature.
Prior stretching processes also include a xe2x80x9ccoldxe2x80x9d process whereby a film is mechanically stretched without heating the film. Cold processes can be in-line or off-line processes. When compared to hot processes, cold processes require a greater stretching or elongating force to stretch the film and the resulting film is stretched less. Cold processes are limited by low draw or stretch ratios, and poor film performance and film appearance.
Because of the limitations encountered during prior art stretching processes, an apparatus and process are needed to produce a highly stretched film at greater processing speeds having high tensile and puncture strength, less neck-in and greater clarity and performance.
The present invention is directed to an apparatus and high speed process for making a highly stretched film. The apparatus and process, which are used for hot stretching, may be used in-line (wherein a film is extruded and stretched before storage or use) or off-line (wherein a film is extruded and stored before stretching). Compared to prior art hot stretching apparatus and processes, the apparatus and process of the invention use fewer and larger draw rollers, operate at higher line speeds with lower draw distance between rollers, and produce films with higher draw ratios and less neck-in.
An apparatus and process according to one preferred embodiment of this invention comprise a bridle mechanism having a first heated roll and a second heated roll for heating and stretching a film.
Preferably, the first heated roll and the second heated roll have an outer diameter of at least 18 inches permitting maximum contact with the film. The first heated roll and second heated roll further comprise an outer mirror surface.
Due to the smoothness of the outer mirror surfaces and the heat transferred from the heated rolls to the film, the film can be fed through the bridle mechanism without slippage. High stretch ratios, up to about 5.5:1, result from increased adhesion of the film to the outer mirror surface of the heated rolls.
In the high speed process of this invention, an extrusion process or supply roll supplies a stretchable film to the apparatus at a rate of up to 1,000 feet per minute (fpm), preferably between 300 fpm and 1,000 fpm. The film is fed between a stretch nip and the first heated roll and adheres to the first heated roll. As the film is passed around the rotating heated rolls, the film is heated. The heated rolls are heated by methods known to those skilled in the art, including internal steam, water, oil and electricity.
Initially, the film passes around the first heated roll. The film then separates from the first heated roll and is drawn to the second heated roll, which rotates at a speed greater than the first heated roll. As a result of the increased rotational speed of the second heated roll, the film is subjected to a stretching or elongating force as the film transfers from the first heated roll to the second heated roll.
Preferably, the closest distance between the first heated roll and the second heated roll is kept to a minimum. The tangential distance between the point where the film separates from the first heated roll and the point where the film contacts the second heated roll, known as the draw gap, preferably is about 0.25 inch to about 1.0 inch. Due to the use of only two heated rolls and the short draw gap, the film can be highly stretched with minimal neck-in. After stretching, the film is passed around the second heated roll.
After leaving the second heated roll, the film is annealed and wound onto a core. The film is allowed to relax (i.e., shrink) up to about 20% of the increase in length due to stretching, preferably in the range of 5% to about 15%, as the film advances between the second heated roll and the core. After relaxing, the film remains up to about 450% longer than its initial, pre-stretched length.
With the foregoing in mind, it is one feature and advantage of this invention to provide an apparatus and a high speed process for heating and stretching a film to a length which is about 100-450% longer than its initial, pre-stretched length.
It is another feature and advantage of this invention to provide an apparatus and a high speed process for heating and stretching a film at a rate of up to about 2,500 fpm.
It is another feature and advantage of this invention to provide an apparatus and process for heating and stretching a film having a minimal number of heated rolls and a small draw gap to maximize contact with the film, and minimize neck-in and slippage of the film on the rolls.
It is yet another feature and advantage of this invention to provide an improved stretched film that is heated and stretched to a length that is about 100-450% longer than its initial pre-stretched length having greater clarity, higher gloss, lower haze, higher tensile and puncture strength, less neck-in and lower noise level.
The foregoing and other features and advantages of this invention will become further apparent from the following detailed description of the preferred embodiments, read in conjunction with the drawing.